100% cotton rug warp from Dorr Mills is my favourite backing for hooking! It holds like linen, is soft on the hands, and has a nice even weave. It is also good when hooking with yarns (as opposed to thicker strips of wool).
Approximately 11 x 13 threads per square inch.
It does like to fray, so I have serged all the edges for you on any of the smaller pieces. When purchased by the yard the edges are not serged, but you can use masking tape to protect the edges once you cut it to your preferred size if you don't have a sewing machine.
I can cut custom sizes as well, just let me know what you need!